An electronic imaging system depends on an electronic image sensor to create an electronic representation of a visual image. Examples of such electronic image sensors include charge coupled device (CCD) image sensors and active pixel sensor (APS) devices (APS devices are often referred to as CMOS sensors because of the ability to fabricate them in a Complementary Metal Oxide Semiconductor process). Typically, these images sensors include a number of light sensitive pixels, often arranged in a regular pattern of rows and columns. For capturing color images, a pattern of filters is typically fabricated on the pattern of pixels, with different filter materials being used to make individual pixels sensitive to only a portion of the visible light spectrum. The color filters necessarily reduce the amount of light reaching each pixel, and thereby reduce the light sensitivity of each pixel. A need persists for improving the light sensitivity, or photographic speed, of electronic color image sensors to permit images to be captured at lower light levels or to allow images at higher light levels to be captured with shorter exposure times.
Image sensors are either linear or two-dimensional. Generally, these sensors have two different types of applications. The two-dimensional sensors are typically suitable for image capture devices such as digital cameras, cell phones and other applications. Linear sensors are often used for scanning documents. In either case, when color filters are employed the image sensors have reduced sensitivity.
A linear image sensor, the KLI-4104 manufactured by Eastman Kodak Company, includes four linear, single pixel wide arrays of pixels, with color filters applied to three of the arrays to make each array sensitive to either red, green, or blue in its entirety, and with no color filter array applied to the fourth array; furthermore, the three color arrays have larger pixels to compensate for the reduction in light sensitivity due to the color filters, and the fourth array has smaller pixels to capture a high resolution luminance image. When an image is captured using this image sensor, the image is represented as a high resolution, high photographic sensitivity luminance image along with three lower resolution images with roughly the same photographic sensitivity and with each of the three images corresponding to either red, green, or blue light from the image; hence, each point in the electronic image includes a luminance value, a red value, a green value, and a blue value. However, since this is a linear image sensor, it requires relative mechanical motion between the image sensor and the image in order to scan the image across the four linear arrays of pixels. This limits the speed with which the image is scanned and precludes the use of this sensor in a handheld camera or in capturing a scene that includes moving objects.
There is also known in the art, an electronic imaging system described in U.S. Pat. No. 4,823,186 by Akira Muramatsu that includes two sensors, wherein each of the sensors includes a two-dimensional array of pixels but one sensor has no color filters and the other sensor includes a pattern of color filters included with the pixels, and with an optical beam splitter to provide each image sensor with the image. Since the color sensor has a pattern of color filters applied, each pixel in the color sensor provides only a single color. When an image is captured with this system, each point in the electronic image includes a luminance value and one color value, and the color image must have the missing colors at each pixel location interpolated from the nearby colors. Although this system improves the light sensitivity over a single conventional image sensor, the overall complexity, size, and cost of the system is greater due to the need for two sensors and a beam splitter. Furthermore, the beam splitter directs only half the light from the image to each sensor, limiting the improvement in photographic speed.
In addition to the linear image sensor mentioned above, there are known in the art, image sensors with two-dimensional arrays of pixels where the pixels include pixels that do not have color filters applied to them. For example, see Sato, et al. in U.S. Pat. No. 4,390,895, Yamagami, et al. in U.S. Pat. No. 5,323,233, Gindele, et al. in U.S. Pat. No. 6,476,865, and Frame in US Patent Application 2003/0210332. In each of the cited patents, the sampling arrangements for the color pixels versus the luminance or unfiltered pixels favor the luminance image over the color image or vice-versa or in some other way provide a suboptimal arrangement of color and luminance pixels.
Therefore, there persists a need for improving the light sensitivity for electronic capture devices that employ a single sensor with a two-dimensional array of pixels.